1. Field of the Invention
The present invention relates to a gallium nitride thin film on sapphire substrate having reduced bending deformation.
2. Description of the Related Art
Generally, a gallium nitride thin film is formed such that a gallium nitride film is grown on a sapphire substrate at a relatively low temperature and the growth is finally completed at a relatively high temperature. However, a bending occurs in the sapphire substrate due to a difference (about 16%) in lattice constants and a difference (about 35%) in thermal expansion coefficients between sapphire and gallium nitride.
FIG. 1 is a graph illustrating a bending range depending on a thickness of a gallium nitride thin film on sapphire substrate according to the related art. Referring to FIG. 1, it can be seen that a curvature radius is reduced as a bending range increases due to an increase of film thickness. Such bending causes the difference in crystalline characteristics between a center portion of a substrate and a periphery of the substrate, wherein a gallium nitride thin film is grown on the substrate. In the case where a light emitting diode is manufactured by using such a substrate, a problem occurs in that a light emitting wavelength becomes uneven in the substrate. Recently, a gallium nitride film, which is used as a lower substrate for a light emitting diode and an electronic device, requires an increase in thickness to reduce defect density and increase crystalline characteristics. However, in this case, problems occur in that uniformity in device quality of a substrate is reduced due to an increase of bending deformation, thus, yield of the product is rapidly reduced.
Accordingly, in order to improve crystalline characteristics and reduce defect density through an increase of film thickness, and to improve characteristics of a light emitting device and an electronic device by increasing electrical reliability, a gallium nitride thin film on sapphire substrate is required, wherein a bending is maintained and does not increase even though the thickness of the gallium nitride film increases.
In this respect, the U.S. Pat. No. 6,528,394 discloses a method of growing an aluminum nitride layer of a concave-convex shaped structure between a gallium nitride layer and a sapphire substrate to reduce surface stress between the gallium nitride layer and the sapphire substrate, wherein the surface stress causes bending deformation. The Korean Patent Application No. 2004-5585 discloses a gallium nitride thin film on sapphire substrate having reduced bending deformation through control in size and distribution density of an aluminum nitride having a concave-convex shaped structure.